Olefins and their substituted counterparts (defined herein as, but not limited to, ethylene, propylene, butenes, and mixtures thereof) serve as feedstocks for the production of numerous chemicals and polymers. For example, ethylene is one of the largest volume chemical intermediates in the world, being used as a raw material in the production of, for example, polyethylene, ethylbenzene-styrene, ethylene dichloride, ethylene oxide and ethylene glycol. Most olefins are commercially produced by the thermal or catalytic cracking of saturated hydrocarbons found in petroleum and naphtha (See M. Ladisch et al., Science (1979) 205, 898). Due to the thermodynamic limitations of the reaction, thermal cracking reactors operate at temperatures as high as 1,100° C. to maintain the desired levels of conversion—typical yields are between 50 and 100% (See U.S. patent applications and patents: 2006/0149109; U.S. Pat. Nos. 4,351,732; 4,556,460; 4,423,270; and 4,134,926). Information on production of ethylene by thermal cracking is available in Kirk Othmer Encyclopedia of Chemical Technology, 5th ed. Wiley (2004-2007) and Ullmann's Encyclopedia of Industrial Chemistry, 6th ed. Wiley (2003), both hereby incorporated by reference.
Finding new, more efficient, and environmentally friendly pathways to produce olefins from renewable starting materials that are not encumbered by the varying costs and tightening supply of crude petroleum has been a challenging research area of the past decade (See U.S. patent applications and patents: 2006/0149109; U.S. Pat. Nos. 4,351,732; 4,556,460; 4,423,270; and 4,134,926). Catalytic oxidative dehydration of ethane was proposed as an alternative method to produce ethylene at much lower temperatures, but the yields and selectivity achieved to date have not been encouraging (See S. Golay et al., Chem. Eng. Sci. (1999) 54, 3593).
Dehydration of oxygenates are conventionally carried out using either concentrated sulfuric acid or concentrated phosphoric acid, H3PO4. The mechanistic details for the dehydration reaction can be summarized in Scheme 1 (below). The alcohol is first protonated, followed by a loss of water to give a carbocation (carbonium ion), which results in the subsequent abstraction of a hydrogen ion from the carbocation. Apart from the acid's corrosive nature, as a side reaction, the acid can oxidize the alcohol into polluting carbon dioxide. Also, in the case of concentrated H2SO4, large quantities of sulfur dioxide can be produced. Both of these gases have to be removed from the product olefin before it can be used in a later chemical process.

Silicoaluminophosphates (SAPOs), such as SAPO-34 and its analogues, possess strong Brönsted acid sites and are excellent shape-selective catalysts for the conversion of methanol and other alcohols to light olefins (See U.S. Pat. Nos. 4,499,327; 5,952,538; 6,046,673; 6,334,994; and 7,199,277; as well as WO 1993/024430). However, SAPOs are composed of Si atoms tetrahedrally coordinated to oxygen atoms making an integral part of the overall catalyst framework. SAPO-34 is being commercially exploited (by UOP) for the selective conversion of methanol to low-molecular weight olefins (See WO 2007/032899). Further, the Brönsted acidity of a SAPO varies greatly depending on its particular structure type and architecture.
To vary the intensity and number of Brönsted acid sites in aluminophosphates (AlPOs), one can isomorphously introduce ions to replace a portion of the AlIII ions with a single type of divalent metal ion, such as Zn, Mg, Mn, Co, Ni, Cu, and Fe, among others. In other words, a fraction of the ≡AlIII—O—PV≡ linkages is replaced by ≡MII-O(H)—PV≡, the proton that is loosely attached to the bridging oxygen being the locus of the Brönsted acid center. The properties of the resulting Brönsted acid center can be controlled by the appropriate choice of structure-directing agents, transition-metal precursor, or gel composition, leading to a wide range of solid-acid catalysts. However, by substituting only the AlIII ion or the PV ion in the framework, only partial tuning of the acid strength occurs. See, for example, EP0141662A2. Yet, phase purity must also be accomplished to get significant selectivity towards the desired product of the reaction.
Olefins, particularly light olefins, are the most desirable products from oxygenate conversion and crude petroleum cracking. A need exists to improve the performance of ethylene plants. To this end, a number of catalytically mediated processes have been proposed. The most chemically straightforward among these is ethanol dehydration. This invention meets that need and provides the full tuning of the AlPO acid strength via controlled, judicious, and simultaneous substitution of both the AlIII and PV ions combined with substantial phase purity of the calcined catalyst.